Forensic Science

The concept of Neutron Activation Analysis (NAA) of barium and antimony ... and filter paper swabs concludes (2) that both techniques are ... A purely...
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10 Progress in Firearm Residue Detection EDGARS RUDZITIS Illinois B u r e a u of Identification, Joliet, Ill. 60432 MORRIS A. WAHLGREN

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A r g o n n e N a t i o n a l L a b o r a t o r y , A r g o n n e , Ill. 60439

The concept of Neutron A c t i v a t i o n A n a l y s i s (NAA) of barium and antimony present in primers as a means of f i r e a r m residue d e t e c t i o n is now ten years o l d and it appears appropriate to point out the s a l i e n t p o i n t s of the development of the technique. The o r i g i n a l work (1) and l a t e r i n v e s t i g a t i o n by high speed photography and autoradiography (2) conclude that Ba from primer and b u l l e t are deposited on the back of the firing hand as d i s c r e t e p a r t i c u l a t e matter. The obvious problem f o r the detec- t i o n of firearms residue on s k i n i s the e f f i c i e n c y of c o l l e c t i o n both from the standpoint of convenience and q u a n t i t a t i o n . A f t e r e x p l o r i n g a v a r i e t y of methods, p a r a f f i n cast was suggested as the best (3) while another i n v e s t i g a t i o n suggests a complete hand r i n s e (4) as the most q u a n t i t a t i v e technique. C o l l o d i o n lifts have been reported (5) and a comparison between c o l l o d i o n lifts and filter paper swabs concludes (2) that both techniques are about e q u a l l y efficient. Sampling by moistened c o t t o n swab has been a l s o suggested (5,6) as a convenient technique. While there has been considerable experimentation with sampling techniques, there have been few attempts to improve the a n a l y t i c a l procedure. The o r i g i n a l separation method (1) has undergone only minor changes (6-8). There have been two attempts to optimize the radiochemical procedure and improve the accuracy by i n t r o d u c i n g Ba and Sb as t r a c e r s (9,10) and by r e - e x p l o r a t i o n of Sb radiochemistry (10). A p u r e l y instrumental method has been reported (11) at the expense of excluding the determination of barium. The b i v a r i a t e - l o g - n o r m a l a n a l y s i s of data c o l l e c t e d by Guinn and co-workers appears to be the only comprehensive statistical treatment of f i r e a r m residue d e t e c t i o n by NAA (11). Suspects' handswabs were i n t e r p r e t e d i n terms of accumulated firing t e s t data and handblanks c o l l e c t e d from i n d i v i d u a l s of d i f f e r e n t occu- p a t i o n a l backgrounds. A somewhat more e m p i r i c a l i n t e r p r e t a t i o n of the same data is a l s o reported (12). A d d i t i o n a l data from smaller s c a l e c o l l e c t i o n of handblanks have been published r e c e n t l y (13,14).

88

In Forensic Science; Davies, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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The NAA method f o r the determination of f i r e a r m discharge residue has been g e n e r a l l y accepted, but a p p l i c a t i o n s have been l i m i t e d to j u s t a few l a b o r a t o r i e s . In the process of e s t a b l i s h ing NAA c a p a b i l i t y f o r the State of I l l i n o i s crime l a b o r a t o r i e s we re-examined the standard techniques (10). In the course of our work i t became c l e a r that p o s t - i r r a d i a t i o n i s the cause of s e v e r a l c o n s t r a i n t s which have discouraged a more widespread use of NAA. The inherent time l i m i t a t i o n due to the 87 min. h a l f - l i f e of B a n e c e s s i t a t e s f a s t manipulations of r a d i o a c t i v e s o l u t i o n s which i n turn r e q u i r e s an experienced radiochemist. In a d d i t i o n to an ever present danger of overexposure and contamination, t y p i c a l l y only a dozen samples can be i r r a d i a t e d per batch, which makes the method q u i t e expensive. The developed s t a t i s t i c a l b i v a r i a t e - n o r m a l a n a l y s i s (11) i s convenient f o r r o u t i n e a p p l i c a t i o n s . With t h i s i n mind, a method was developed which: a) e l i m i n a t e s p o s t - i r r a d i a t i o n r a d i o c h e m i s t r y and thus maximizes time f o r a n a l y s i s ; b) accommodates over 130 samples per i r r a d i a t i o n capsule ( r a b b i t ) ; c) does not r e q u i r e a c o l l e c t i o n of o c c u p a t i o n a l handblanks; and d) u t i l i z e s a s i m p l i f i e d s t a t i s t i c a l concept based on n a t u r a l antimony and barium l e v e l s on hands f o r the i n t e r p r e t a t i o n of data. The d e t a i l e d procedure w i l l be published elsewhere (15). B r i e f l y , a cotton swab technique i s employed; a f t e r swabbing Sb and Ba are f i x e d on the c o t t o n of the swab by moistening i t w i t h an aqueous s o l u t i o n of thioacetamide (TAA) and then w i t h d i l u t e s u l f u r i c a c i d . The cotton of the swab i s then s t r i p p e d and s o l u b l e i n t e r f e r r i n g m a t e r i a l s , n o t a b l y NaCl, are removed by l e a c h i n g with 2N HgSO^ and methanol. The t r e a t e d c o t t o n i s compacted i n t o a small p i e c e of %" polyethylene tubing and i r r a d i a t e d a t a neutron f l u x of 5x10 N/cm /sec. Best r e s u l t s were obtained by double i r r a d i a t i o n : 40 min. with immediate count f o r barium and 5 h r s with 2 day c o o l i n g f o r antimony. The photo peaks of Ba and S b were determined u t i l i z i n g a Ge-Li d e t e c t o r with a computerized pulseheight a n a l y s i s system. With the a v a i l a b i l i t y of some 50 s e t s of handblanks ( e n v i r onmental-natural l e v e l s of Ba and Sb on hands), f i r i n g t e s t s and c a l i b r a t i o n s , we considered a d i f f e r e n t concept f o r the i n t e r p r e t a t i o n of the r e s u l t s . The e v a l u a t i o n c o n s i s t e d of two s t e p s : 1) e s t a b l i s h i n g t h a t the Ba and Sb v a l u e s of handblanks of the accumulated p o p u l a t i o n sample followed a normal (Gaussian) d i s t r i b u t i o n as s t a t i s t i c a l l y approximated by the t - D i s t r i b u t i o n , and 2) u t i l i z a t i o n of r e l a t i v e l y simple s t a t i s t i c a l formalism f o r the c a l c u l a t i o n of the p r o b a b i l i t y t h a t the amount of Ba and Sb found on a given swab belongs to the e s t a b l i s h e d handblank populat i o n . (An appendix at the end of the paper may be u s e f u l to readers not normally u t i l i z i n g s t a t i s t i c s ) .

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1 3 9

>

12

1 3 9

1 2 2

In Forensic Science; Davies, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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SCIENCE

PRESENTATION OF DATA

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Calibration In order to t e s t the r e l i a b i l i t y of the chemical and i r r a d i a t i o n procedure, d e t e c t i o n , and data r e d u c t i o n d e s c r i b e d i n r e f e r e n c e (15), each i r r a d i a t i o n batch had s e v e r a l s e t s of chemical standards. In Table 1, " i n " represents the amounts i n nanograms, ng, of Sb and Ba standards deposited on swabs, the mean of 12 d e t e r minations each i s i n the "found" column and the standard d e v i a t i o n , " s t d . dev.", i s given as a percentage of the "found" v a l u e . The average value i n terms of CPM (counts per minute) per ng c a l c u l a t ed from the three concentrations (5,10 and 20ng) y i e l d e d 98 CPM ± 27% f o r Ba and was used as a b a s i s to c a l c u l a t e "found" v a l u e s . TABLE I CALIBRATION DATA (nanograms) Sb in

found

5 10 20

5 8 22

Ba std.dev. ( % )

a

in

27 18 32

50 100 200

std.dev. expressed i n % of the

found

49 101 270

Std.dev.(%)

a

22 14 25

"found" v a l u e .

G r a p h i c a l e x t r a p o l a t i o n y i e l d e d l i n e a r p l o t s through the o r i g i n w i t h i n a corresponding one standard d e v i a t i o n range f o r each concentration. (Previous t r a c e r experiments have shown a v i r t u a l 100% r e t e n t i o n of *Sb and B a on c o t t o n , (10)) . Considering the involved path from sampling through data reduct i o n , the method appears to be s u f f i c i e n t l y r e l i a b l e t o warrant a s t a t i s t i c a l analysis. 121

1 3 3

Handblank Values Table 2 represents the mean values and standard d e v i a t i o n s i n percent of the mean value of the f o u r separate hand swabs, r i g h t back, (RB), r i g h t palm, (RP), l e f t back (LB), l e f t palm (LP), c o l l e c t e d from 30 persons. The hands of crime l a b t e c h n i c i a n s and p o l i c e o f f i c e r s were swabbed "as i s " without any pretreatment. While there were o c c a s i o n a l higher readings on palms, the found values were a l l represented by the means and standard d e v i a t i o n s . In only one instance were high Ba values ( l e s s than 1:333 p r o b a b i l i t y being ^normal ') encountered on a l l four s u r f a c e s . when q u e s t i o n 1

In Forensic Science; Davies, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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ed, the i n d i v i d u a l remembered having used a carbon-based s o l i d l u b r i c a n t the night before, which was d i f f i c u l t t o remove from the hands and t h e r e f o r e q u i t e l i k e l y was the o r i g i n of the high Ba determination; n e v e r t h e l e s s , these values weiealso included i n the c a l c u l a t i o n of the means. As can be seen from the t a b l e , the mean values of the handblanks d i d not d i f f e r s i g n i f i c a n t l y i n terms of standard d e v i a t i o n s and i t was convenient to lump them as the t o t a l hand averages of 0.4 and 7 ng of Sb and Ba r e s p e c t i v e l y , w i t h an approximated standard d e v i a t i o n of ± 100%, i . e . , ± 0.4 and t 7 ng r e s p e c t i v e l y . While our values are lower than those encountered by previous workers,the r e l a t i v e r e l a t i o n s h i p s are e s s e n t i a l l y unchanged and the data are s e l f - c o n s i s t e n t . TABLE I I HANDBLANK VALUES (nanograms) Antimony

Barium

RB

RP

LB

LP

RB

RP

LB

LP

mean (ng)

0.4

0.5

0.3

0.4

6

8

5

10

std.dev.(%)

81

116

87

110

116

87

128

93

t - D i s t r i b u t i o n of Sb and Ba Values of

Handblanks

There i s no a p r i o r i reason to doubt that the C e n t r a l L i m i t Theorem, and consequently the normal d i s t r i b u t i o n concept, app l i e s to t r a c e element d i s t r i b u t i o n , i n c l u d i n g Sb and Ba on hands i n a human population, because these c o n c e n t r a t i o n s are a f f e c t e d by such random v a r i a b l e s as l o c a t i o n , d i e t , metabolism, and so on. However, s i n c e enough data were at hand (some 120 samples per element), i t was of i n t e r e s t to t e s t the normal d i s t r i b u t i o n experimentally by examination of the t - D i s t r i b u t i o n . The probab i l i t y d e n s i t y p l o t s of 0.2 and 3 ng increments f o r Sb and Ba, r e s p e c t i v e l y , had s i m i l a r appearances. The a c t u a l d i s t r i b u t i o n t e s t was c a r r i e d out f o r Sb only because of b e t t e r data due t o the more convenient h a l f l i f e of S b . After normalization, a "one t a i l " t e s t was c a r r i e d out. Normalization c o n s i s t e d of s h i f t i n g the s c a l e one standard d e v i a t i o n to the l e f t so that the o r i g i n and maximum (mean) were superimposed. A l s o , the v e r t i c a l s c a l e was s h i f t e d i n order to superimpose the t h e o r e t i c a l and experimental p r o b a b i l i t y d e n s i t i e s at the o r i g i n . The t h e o r e t i c a l values were c a l c u l a t e d from the expression f ( Z ) = (2TT) ^ / e ^ d z where z= (x.-x)/s=x/s ( f o r d e f i n i 1 2 2

z 2

In Forensic Science; Davies, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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t i o n s see Appendix) s i n c e x=o as a r e s u l t o f the s h i f t . The t h e o r e t i c a l and experimental values are compared i n Table 3 as a f u n c t i o n of weight increments of antimony. As can be seen, the agreement i s as good as can be expected f o r an average o f 13 samples per weight increment. The experimental values show the t a i l expected f o r a t - D i s t r i b u t i o n . Therefore, the a p p l i c a t i o n of t - t e s t s to determine the p r o b a b i l i t y o f a given antimony amount of a hand swab being n a t u r a l background i s j u s t i f i e d . By analogy, the same c o n c l u s i o n may be made f o r barium l e v e l s . TABLE 3 EXPERIMENTAL t-DISTRIBUTION VS. NORMAL DISTRIBUTION FOR Sb HANDBLANKS x (ng Sb)

Theoretical

Experimental

42 32 19 9 3 1 0

42 32 15 14 7 5 4 0 1

0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7

Comparison of Handblanks and F i r i n g T e s t s . T h i r t e e n known one hand-one shot f i r i n g s of both automatics and r e v o l v e r s o f v a r i o u s c a l i b e r s were s e l e c t e d . The means and ranges o f antimony and barium values f o r the f i r i n g hand a r e presented i n Table 4. TABLE 4 FIRING TEST DATA FIRING HAND BACK (FHB) Sb

a

FIRING HAND PALM (FHP)

Ba

Sb

Ba

6.6

68

0.4-26

0-223

mean:

10.0

144

range:

2.5-27

27-660

a A l l u n i t s are ng.

In Forensic Science; Davies, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

10.

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WAHLGREN

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Detection

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I f the means are taken as a c h a r a c t e r i s t i c t e s t f i r i n g , i t i s obvious from the range that standard d e v i a t i o n has l i t t l e meaning f o r comparison with handblanks - one of the d i f f i c u l t i e s encountered i previous i n v e s t i g a t i o n s when s t a t i s t i c s were based on t e s t f i r i n g s (11, 12). An i n t e r e s t i n g note i s that the f i r i n g hand back, FHB, has roughly twice the amount of Sb and Ba as the palm, FHP, which i n turn shows about ten times the amounts of a t y p i c a l handblank. In order to q u a n t a t i v e l y compare the means of the f i r i n g t e s t s and those of handblanks, we w i l l use the so c a l l e d N u l l Hypothesis i n conjunction with the t - T e s t . The N u l l Hypothesis d i v i d e s the expected r e s u l t s i n t o two c l a s s e s , the acceptance (true) c l a s s and the r e j e c t i o n ( f a l s e ) c l a s s , i n such a way t h a t the p r o b a b i l i t y of the r e j e c t i o n c l a s s when the N u l l Hypothesis i s true i s equal to some small preassigned value c a l l e d the s i g n i f i c a n c e l e v e l . In t h i s i n s t a n c e , the hypothesis w i l l be that the f i r i n g t e s t data represent handblanks. (This i s not an unreasonable assumption, because whenever a handswab i s taken i n a case i t has to be assumed that the s u b j e c t has a normal handblank u n t i l proved o t h e r w i s e ) . In other words, because of p o t e n t i a l l y s e r i o u s i m p l i c a t i o n s , every attempt should be made to avoid the e r r o r of r e j e c t i n g the N u l l Hypothesis (conc l u s i o n of abnormally l a r g e Sb and Ba amounts) when a c t u a l l y the hypothesis i s true (normal handblanks). U t i l i z i n g the t - D i s t r i b u t i o n t a b l e s of Bulmer s t e x t (16) (see Appendix), the most conservative l i m i t was chosen namely a 1:2000 p r o b a b i l i t y of r e j e c t i n g the N u l l Hypothesis when a c t u a l l y t r u e ( i . e . , r e j e c t ing the assumption of normal handblanks). The degree of freedom was taken as 120 ( a c t u a l l y 119) because, as Table 2 shows, the combination of a l l four data f o r a handblank i s j u s t i f i e d . Table 5 i l l u s t r a t e s the t - t e s t by comparing the mean Sb and Ba values of f i r i n g t e s t s and handblanks. The wide disagreement between the c a l c u l a t e d and t h e o r e t i c a l i s not s u r p r i s i n g i f one considers that i n a normal d i s t r i b u t i o n there i s a 1:333 probab i l i t y that a value i s o u t s i d e three standard d e v i a t i o n s , whereas i n t h i s example even f i r i n g hand palm values are approximately 10 standard d e v i a t i o n s away i n terms of mean and standard d e v i a t i o n of the handblanks. The i m p r o b a b i l i t y of matching the data of handblanks and f i r i n g t e s t s i s made even more astronomical by the m u l t i p l i c a t i o n r u l e of s t a t i s t i c s because both elements, Sb and Ba, have to be considered. A p o s i t i v e c o r r e l a t i o n between back and palm values c e r t a i n l y should have an added s t a t i s t i c a l s i g n i f i c a n c e as to the abnormally high antimony and barium values of the f i r i n g t e s t s .

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n

1

f

CONCLUSIONS I t should be reemphasized that the complete i n c o m p a t i b i l i t y of f i r i n g t e s t and handblank values are based s t r i c t l y on the s t a t i s tical treatment of the two sets of data accumulated so f a r . A more general a p p l i c a b i l i t y w i l l h o p e f u l l y emerge as more hand-

In Forensic Science; Davies, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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blanks are accumulated. I t can be r e a l i s t i c a l l y argued, though, that environmental handblanks of sharpshooters, employees of fireworks f a c t o r i e s , or s c i e n t i s t engaged i n research of Sb-Ba a l l o y s may be s i g n i f i c a n t l y h i g h e r . While the above s t a t i s t i c a l reasoning may not yet be a p p l i e d w i t h i n the present j u d i c i a l system, i t at l e a s t provides a backup f o r reaching d e c i s i o n s . A f t e r a l l , the c o r r e c t n e s s of the judgment of an expert witness i s a l s o subject to the laws of statistics. APPENDIX Sample, x i s a s i n g l e measurement of a datum from a population. ** ** Mean and average, x, can be used i n t e r changeably. The standard d e v i a t i o n , s, i s a u s e f u l parameter f o r a reasonably l a r g e p o p u l a t i o n ; n, i s , i n e f f e c t , an absolute average d e v i a t i o n from the mean. The degree of freedom i s n-1 ( i . e . , with n=l no s t a t i s t i c s i s p o s s i b l e ) . The p r o b a b i l i t y d e n s i t y i s the percentage of samples w i t h i n a given range. Normal and t - D i s t r i b u t i o n s : the Normal D i s t r i b u t i o n i s r e p r e sented by the well-known bell-shaped curve, where the maximum i s represented by the mean, x, and the standard d e v i a t i o n , s, i s the width at the i n f l e c t i o n p o i n t . The b a s i s of i t s a p p l i c a t i o n to many n a t u r a l phenomena i s the C e n t r a l - L i m i t Theorem, which s t a t e s that the sum of a l a r g e number of independent v a r i a b l e s w i l l be approximately normally d i s t r i b u t e d r e g a r d l e s s of t h e i r i n d i v i d u a l d i s t r i b u t i o n s . The theory of the normal d i s t r i b u t i o n was d e v e l oped from a l a r g e number of samples, which, of course, can not be s t r i c t l y a p p l i c a b l e i n most p r a c t i c a l cases. T h i s was recognized e a r l y by an I r i s h chemist W.S. Gosset (17) who formed the b a s i s of of the "small sample theory" - the so c a l l e d t - D i s t r i b u t i o n - i n c l u d i n g - t - T e s t . The theory, i n e f f e c t , s t a t e s that the t - D i s t r i but ion has the same shape asthe normal d i s t r i b u t i o n except that the curve i s f l a t t e r and has a longer t a i l . The t-Test,which i s an estimate of the p r o b a b i l i t y by which a given sample f a l l s i n t o an e s t a b l i s h e d t - D i s t r i b u t i o n , i s represented by the formula t = ( x - x / s ) n 2 , where x,^ i s the value of a given sample and x, s, 9

n

x

i

and n are the parameters of a p r e v i o u s l y e s t a b l i s h e d t - D i s t r i b u t i o n . The c a l c u l a t e d t-value i s compared with tabulated t h e o r e t i c a l values and the p r o b a b i l i t y of f i t (or m i s f i t ) i s thus d e t e r mined . The "One t a i l " , i n which only one h a l f of the curve i s t e s t e d , i s i n d i c a t e d when below-average values are not of i n t e r e s t or when the s e n s i t i v i t y i s l i m i t e d (as i n very low l e v e l antimony, barium d e t e r m i n a t i o n s ) . The "small sample s t a t i s t i c s " i s widely used and has obvious a p p l i c a t i o n s i n c r i m i n a l i s t i c s . A number of s u i t a b l e t e x t s are a v a i l a b l e (16,18).

In Forensic Science; Davies, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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TABLE 5 ILLUSTRATION OF THE t-TEST Sb x

(FHB)

10.0 ng

128 ng

x (FHP)

6.3 ng

68 ng

X

0.4 ng

7 ng

s

0.4 ng

7 ng

±

±

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Ba

t (FHB)

190

143

Tabulated t-value ( a p p l i e s both to Sb and Ba) f o r m=120 and at a s i g n i f i c a n c e l e v e l of 0.0005 i s 3.4; the corresponding t-value f o r a s i g n i f i c a n c e l e v e l 0.01 i s 2.4. FHB=firing hand back, FHP=firing hand palm.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Rush, R.R., Guinn, V.P. and Pinker, R.H., NUCL. SCI AND ENG., (1964), 22, 381. S c o t t , H.D., Coleman, R.F., and C r i p s , F.H., UKAEA, AWRE REPORT 0-5/66. Bryan, D.E. and Guinn, V.P., GULF G.A. REPORT GA-5556, (1964). Krishnan, S.S., J. CAN. SOC. FOR. SCI., (1973), 6, 55. Albu-Yaron, A. and Amiel, S., J. RADIOANAL. CHEM., (1972), 11, 123. Hoffman, C.M., IDENT. NEWS, (1968), 18, 7. Pro, M.J., INST. CRIM. POL. REV., (1970), 372, 270. Hoffman, C.M., J.A.O.A.C, (1973) 56, 1388 McFarland, R.C. and McLain, M.E., J. FOR. SCI., (1973), 18, 226 R u d z i t i s , E., Kopina, M. and Wahlgren, M.A., J. FOR. SCI., (1973), 18, 93 S c h l e s i n g e r , H.L., Lukens, H.R., Guinn, V.P., Hackleman, R.P., and K o r t s , R.F., GULF G.A. REPORT 9829, (1970). Lukens, H.R., GULF G.A. REPORT GA-10245, (1970). M i d k i f f , R.C., IDENT. NEWS, (1973), 23, 9. Gerber, S.R., e t . al., 6th INT. SYMP. ON FOR. SCI., Glasgow, Scotland, (1972).

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R u d z i t i s , Ε., and Wahlgren, M.A., to be published in J . FOR. SCI., (1975) Bulmer, M.G., " P r i n c i p l e s of Statistics", M.I.T. Press, Cambridge, Mass., 1965. "Student", BIOMETRIKA (1908), 6, 1. Bauer, E.L., "A Statistical Manual f o r Chemists", Academic Press, N.Y., N.Y., 1974; Mood, A.M. and G r a y b i l l , F.A., " I n t r o d u c t i o n to the Theory of Statistics", McGraw Hill, N.Y., N.Y., 1963.

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